Semiconductor device and methods of forming

ABSTRACT

In an embodiment, a device includes a first fin extending from a substrate. The device also includes a first gate stack over and along sidewalls of the first fin. The device also includes a first gate spacer disposed along a sidewall of the first gate stack. The device also includes and a first source/drain region in the first fin and adjacent the first gate spacer, the first source/drain region including a first epitaxial layer on the first fin, the first epitaxial layer having a first dopant concentration of boron. The device also includes and a second epitaxial layer on the first epitaxial layer, the second epitaxial layer having a second dopant concentration of boron, the second dopant concentration being greater than the first dopant concentration.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No.63/106,154, filed on Oct. 27, 2020, which application is herebyincorporated herein by reference.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as, for example, personal computers, cell phones, digital cameras,and other electronic equipment. Semiconductor devices are typicallyfabricated by sequentially depositing insulating or dielectric layers,conductive layers, and semiconductor layers of material over asemiconductor substrate, and patterning the various material layersusing lithography to form circuit components and elements thereon.

The semiconductor industry continues to improve the integration densityof various electronic components (e.g., transistors, diodes, resistors,capacitors, etc.) by continual reductions in minimum feature size, whichallow more components to be integrated into a given area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates an example of a FinFET in a three-dimensional view,in accordance with some embodiments.

FIGS. 2, 3, 4, 5, 6, 7, 8A, 8B, 9A, 9B, 10A, 10B, 10C, 10D, 11A, 11B,11C, 11D, 12A, 12B, 13A, 13B, 14A, 14B, 15A, 15B, 15C, 16A, 16B, 17A,and 17B are cross-sectional views of intermediate stages in themanufacturing of FinFETs, in accordance with some embodiments.

FIG. 18 is a perspective view of a source/drain region in accordancewith some embodiments.

FIG. 19 is a graph illustrating concentration of dopant of asource/drain region in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Fin Field-Effect Transistors (FinFETs) and methods of forming the sameare provided in accordance with various embodiments. Intermediate stagesof forming FinFETs are illustrated. Some embodiments discussed hereinare discussed in the context of FinFETs formed using a gate-last(sometimes referred to as replacement gate process) process. In otherembodiments, a gate-first process may be used. Some variations of theembodiments are discussed. Also, some embodiments contemplate aspectsused in planar devices, such as planar FETs. One of ordinary skill inthe art will readily understand other modifications that may be madethat are contemplated within the scope of other embodiments. Althoughmethod embodiments are discussed in a particular order, various othermethod embodiments may be performed in any logical order and may includefewer or more steps described herein.

Before addressing the illustrated embodiments specifically, certainadvantageous features and aspects of the present disclosed embodimentswill be addressed generally. In general terms, the present disclosure isa semiconductor device and method of forming the same to improve theperformance of FinFET devices by reducing the parasitic resistance ofthe source/drain regions of the semiconductor device. In the disclosedembodiments, the source/drain regions include an ultra-heavily dopedlayer on the outside of the source/drain region, which can reduce theparasitic resistance of the source/drain region. For example, for p-typemetal-oxide-semiconductor FETs (MOSFETs), such as p-type FinFETs, thesource/drain region includes an outer layer ultra-heavily doped withboron to reduce the parasitic resistance. With the inclusion of theheavily doped layer, the Schottky barrier height at the interfacebetween a conductive contact and the source/drain region is reduced, andthus, a carrier can more easily tunnel through the interface, whichlowers the parasitic resistance. This reduction of the parasiticcapacitance can boost the performance of the semiconductor device.Specifically, the inventors found that for p-typemetal-oxide-semiconductor FETs (MOSFETs), the p-type dopingconcentration in the source/drain region greatly affects the totalresistance of the semiconductor device as the parasitic resistance is alarge portion of the total resistance for p-type MOSFETs. The disclosedprocesses and structures can improve the performance and reliability ofthe FinFET device.

Some embodiments contemplate both n-type devices, such as n-typeFinFETs, and p-type devices, such as p-type FinFETs, being manufacturedduring a manufacturing process. Hence, some embodiments contemplate theformation of complementary devices. Figures below may illustrate onedevice, but one of ordinary skill in the art will readily understandthat multiple devices, some with a different device type, can be formedduring processing. Some aspects of the formation of complementarydevices are discussed below, although such aspects may not necessarilybe illustrated in the figures.

FIG. 1 illustrates an example of a FinFET in a three-dimensional view,in accordance with some embodiments. The FinFET comprises a fin 52 on asubstrate 50 (e.g., a semiconductor substrate). Isolation regions 56 aredisposed in the substrate 50, and the fin 52 protrudes above and frombetween neighboring isolation regions 56. Although the isolation regions56 are described/illustrated as being separate from the substrate 50, asused herein the term “substrate” may be used to refer to just thesemiconductor substrate or a semiconductor substrate inclusive ofisolation regions. Additionally, although the fin 52 is illustrated as asingle, continuous material as the substrate 50, the fin 52 and/or thesubstrate 50 may comprise a single material or a plurality of materials.In this context, the fin 52 refers to the portion extending between theneighboring isolation regions 56.

A gate dielectric layer 92 is along sidewalls and over a top surface ofthe fin 52, and a gate electrode 94 is over the gate dielectric layer92. Source/drain regions 82 are disposed in opposite sides of the fin 52with respect to the gate dielectric layer 92 and gate electrode 94. FIG.1 further illustrates reference cross-sections that are used in laterfigures. Cross-section A-A is along a longitudinal axis of the gateelectrode 94 and in a direction, for example, perpendicular to thedirection of current flow between the source/drain regions 82 of theFinFET. Cross-section B-B is perpendicular to cross-section A-A and isalong a longitudinal axis of the fin 52 and in a direction of, forexample, a current flow between the source/drain regions 82 of theFinFET. Cross-section C-C is parallel to cross-section A-A and extendsthrough a source/drain region of the FinFET. Subsequent figures refer tothese reference cross-sections for clarity.

Some embodiments discussed herein are discussed in the context ofFinFETs formed using a gate-last process. In other embodiments, agate-first process may be used. Also, some embodiments contemplateaspects used in planar devices, such as planar FETs, nanostructure(e.g., nanosheet, nanowire, gate-all-around, or the like) field effecttransistors (NSFETs), or the like.

FIGS. 2 through 17B are cross-sectional views of intermediate stages inthe manufacturing of FinFETs, in accordance with some embodiments. FIGS.2 through 7 illustrate reference cross-section A-A illustrated in FIG. 1, except for multiple fins/FinFETs. FIGS. 8A, 9A, 10A, 11A, 12A, 13A,14A, 15A, 16A, and 17A are illustrated along reference cross-section A-Aillustrated in FIG. 1 , and FIGS. 8B, 9B, 10B, 11B, 12B, 13B, 14B, 15B,15C, 16B, and 17B are illustrated along a similar cross-section B-Billustrated in FIG. 1 , except for multiple fins/FinFETs. FIGS. 10C,10D, 11C, and 11D are illustrated along reference cross-section C-Cillustrated in FIG. 1 , except for multiple fins/FinFETs.

In FIG. 2 , a substrate 50 is provided. The substrate 50 may be asemiconductor substrate, such as a bulk semiconductor, asemiconductor-on-insulator (SOI) substrate, or the like, which may bedoped (e.g., with a p-type or an n-type dopant) or undoped. Thesubstrate 50 may be a wafer, such as a silicon wafer. Generally, an SOIsubstrate is a layer of a semiconductor material formed on an insulatorlayer. The insulator layer may be, for example, a buried oxide (BOX)layer, a silicon oxide layer, or the like. The insulator layer isprovided on a substrate, typically a silicon or glass substrate. Othersubstrates, such as a multi-layered or gradient substrate may also beused. In some embodiments, the semiconductor material of the substrate50 may include silicon; germanium; a compound semiconductor includingsilicon carbide, gallium arsenide, gallium phosphide, indium phosphide,indium arsenide, and/or indium antimonide; an alloy semiconductorincluding silicon-germanium, gallium arsenide phosphide, aluminum indiumarsenide, aluminum gallium arsenide, gallium indium arsenide, galliumindium phosphide, and/or gallium indium arsenide phosphide; orcombinations thereof.

The substrate 50 has an n-type region 50N and a p-type region 50P. Then-type region 50N can be for forming n-type devices, such as NMOStransistors, e.g., n-type FinFETs. The p-type region 50P can be forforming p-type devices, such as PMOS transistors, e.g., p-type FinFETs.The n-type region 50N may be physically separated from the p-type region50P (as illustrated by divider 51), and any number of device features(e.g., other active devices, doped regions, isolation structures, etc.)may be disposed between the n-type region 50N and the p-type region 50P.

In FIG. 3 , fins 52 are formed in the substrate 50. The fins 52 aresemiconductor strips. In some embodiments, the fins 52 may be formed inthe substrate 50 by etching trenches in the substrate 50. The etchingmay be any acceptable etch process, such as a reactive ion etch (RIE),neutral beam etch (NBE), the like, or a combination thereof. The etchmay be anisotropic.

The fins may be patterned by any suitable method. For example, the fins52 may be patterned using one or more photolithography processes,including double-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins. In some embodiments, the mask (or other layer) may remain on thefins 52.

In FIG. 4 , an insulation material 54 is formed over the substrate 50and between neighboring fins 52. The insulation material 54 may be anoxide, such as silicon oxide, a nitride, the like, or a combinationthereof, and may be formed by a high density plasma chemical vapordeposition (HDP-CVD), a flowable CVD (FCVD) (e.g., a CVD-based materialdeposition in a remote plasma system and post curing to make it convertto another material, such as an oxide), the like, or a combinationthereof. Other insulation materials formed by any acceptable process maybe used. In the illustrated embodiment, the insulation material 54 issilicon oxide formed by a FCVD process. An anneal process may beperformed once the insulation material is formed. In an embodiment, theinsulation material 54 is formed such that excess insulation material 54covers the fins 52. Although the insulation material 54 is illustratedas a single layer, some embodiments may utilize multiple layers. Forexample, in some embodiments a liner (not shown) may first be formedalong a surface of the substrate 50 and the fins 52. Thereafter, a fillmaterial, such as those discussed above may be formed over the liner.

In FIG. 5 , a removal process is applied to the insulation material 54to remove excess insulation material 54 over the fins 52. In someembodiments, a planarization process such as a chemical mechanicalpolish (CMP), an etch-back process, combinations thereof, or the likemay be utilized. The planarization process exposes the fins 52 such thattop surfaces of the fins 52 and the insulation material 54 are levelafter the planarization process is complete. In embodiments in which amask remains on the fins 52, the planarization process may expose themask or remove the mask such that top surfaces of the mask or the fins52, respectively, and the insulation material 54 are level after theplanarization process is complete.

In FIG. 6 , the insulation material 54 is recessed to form ShallowTrench Isolation (STI) regions 56. The insulation material 54 isrecessed such that upper portions of fins 52 in the n-type region 50Nand in the p-type region 50P protrude from between neighboring STIregions 56. Further, the top surfaces of the STI regions 56 may have aflat surface as illustrated, a convex surface, a concave surface (suchas dishing), or a combination thereof. The top surfaces of the STIregions 56 may be formed flat, convex, and/or concave by an appropriateetch. The STI regions 56 may be recessed using an acceptable etchingprocess, such as one that is selective to the material of the insulationmaterial 54 (e.g., etches the material of the insulation material 54 ata faster rate than the material of the fins 52). For example, an oxideremoval using, for example, dilute hydrofluoric (dHF) acid may be used.

The process described with respect to FIGS. 2 through 6 is just oneexample of how the fins 52 may be formed. In some embodiments, the finsmay be formed by an epitaxial growth process. For example, a dielectriclayer can be formed over a top surface of the substrate 50, and trenchescan be etched through the dielectric layer to expose the underlyingsubstrate 50. Homoepitaxial structures can be epitaxially grown in thetrenches, and the dielectric layer can be recessed such that thehomoepitaxial structures protrude from the dielectric layer to formfins. Additionally, in some embodiments, heteroepitaxial structures canbe used for the fins 52. For example, the fins 52 in FIG. 5 can berecessed, and a material different from the fins 52 may be epitaxiallygrown over the recessed fins 52. In such embodiments, the fins 52comprise the recessed material as well as the epitaxially grown materialdisposed over the recessed material. In an even further embodiment, adielectric layer can be formed over a top surface of the substrate 50,and trenches can be etched through the dielectric layer. Heteroepitaxialstructures can then be epitaxially grown in the trenches using amaterial different from the substrate 50, and the dielectric layer canbe recessed such that the heteroepitaxial structures protrude from thedielectric layer to form the fins 52. In some embodiments wherehomoepitaxial or heteroepitaxial structures are epitaxially grown, theepitaxially grown materials may be in situ doped during growth, whichmay obviate prior and subsequent implantations although in situ andimplantation doping may be used together.

Still further, it may be advantageous to epitaxially grow a material inn-type region 50N (e.g., an NMOS region) different from the material inp-type region 50P (e.g., a PMOS region). In various embodiments, upperportions of the fins 52 may be formed from silicon-germanium(Si_(x)Ge_(1-x), where x can be in the range of 0 to 1), siliconcarbide, pure or substantially pure germanium, a III-V compoundsemiconductor, a II-VI compound semiconductor, or the like. For example,the available materials for forming III-V compound semiconductorinclude, but are not limited to, indium arsenide, aluminum arsenide,gallium arsenide, indium phosphide, gallium nitride, indium galliumarsenide, indium aluminum arsenide, gallium antimonide, aluminumantimonide, aluminum phosphide, gallium phosphide, and the like.

Further in FIG. 6 , appropriate wells (not shown) may be formed in thefins 52 and/or the substrate 50. In some embodiments, a P well may beformed in the n-type region 50N, and an N well may be formed in thep-type region 50P. In some embodiments, a P well or an N well are formedin both the n-type region 50N and the p-type region 50P.

In the embodiments with different well types, the different implantsteps for the n-type region 50N and the p-type region 50P may beachieved using a photoresist and/or other masks (not shown). Forexample, a photoresist may be formed over the fins 52 and the STIregions 56 in the n-type region 50N. The photoresist is patterned toexpose the p-type region 50P of the substrate 50. The photoresist can beformed by using a spin-on technique and can be patterned usingacceptable photolithography techniques. Once the photoresist ispatterned, an n-type impurity implant is performed in the p-type region50P, and the photoresist may act as a mask to substantially preventn-type impurities from being implanted into the n-type region 50N. Then-type impurities may be phosphorus, arsenic, antimony, or the likeimplanted in the region to a concentration of equal to or less than 10¹⁸cm⁻³, such as between about 10¹⁶ cm⁻³ and about 10¹⁸ cm⁻³. After theimplant, the photoresist is removed, such as by an acceptable ashingprocess.

Following the implanting of the p-type region 50P, a photoresist isformed over the fins 52 and the STI regions 56 in the p-type region 50P.The photoresist is patterned to expose the n-type region 50N of thesubstrate 50. The photoresist can be formed by using a spin-on techniqueand can be patterned using acceptable photolithography techniques. Oncethe photoresist is patterned, a p-type impurity implant may be performedin the n-type region 50N, and the photoresist may act as a mask tosubstantially prevent p-type impurities from being implanted into thep-type region 50P. The p-type impurities may be boron, boron fluoride,indium, or the like implanted in the region to a concentration of equalto or less than 10¹⁸ cm⁻³, such as between about 10¹⁶ cm⁻³ and about10¹⁸ cm⁻³. After the implant, the photoresist may be removed, such as byan acceptable ashing process.

After the implants of the n-type region 50N and the p-type region 50P,an anneal may be performed to repair implant damage and to activate thep-type and/or n-type impurities that were implanted. In someembodiments, the grown materials of epitaxial fins may be in situ dopedduring growth, which may obviate the implantations, although in situ andimplantation doping may be used together.

In FIG. 7 , a dummy dielectric layer 60 is formed on the fins 52. Thedummy dielectric layer 60 may be, for example, silicon oxide, siliconnitride, a combination thereof, or the like, and may be deposited orthermally grown according to acceptable techniques. A dummy gate layer62 is formed over the dummy dielectric layer 60, and a mask layer 64 isformed over the dummy gate layer 62. The dummy gate layer 62 may bedeposited over the dummy dielectric layer 60 and then planarized, suchas by a CMP. The mask layer 64 may be deposited over the dummy gatelayer 62. The dummy gate layer 62 may be a conductive or non-conductivematerial and may be selected from a group including amorphous silicon,polycrystalline-silicon (polysilicon), poly-crystallinesilicon-germanium (poly-SiGe), metallic nitrides, metallic silicides,metallic oxides, and metals. The dummy gate layer 62 may be deposited byphysical vapor deposition (PVD), CVD, sputter deposition, or othertechniques for depositing the selected material. The dummy gate layer 62may be made of other materials that have a high etching selectivity fromthe etching of isolation regions, e.g., the STI regions 56 and/or thedummy dielectric layer 60. The mask layer 64 may include one or morelayers of, for example, silicon nitride, silicon oxynitride, or thelike. In this example, a single dummy gate layer 62 and a single masklayer 64 are formed across the n-type region 50N and the p-type region50P. It is noted that the dummy dielectric layer 60 is shown coveringonly the fins 52 for illustrative purposes only. In some embodiments,the dummy dielectric layer 60 may be deposited such that the dummydielectric layer 60 covers the STI regions 56, extending over the STIregions and between the dummy gate layer 62 and the STI regions 56.

FIGS. 8A through 17B illustrate various additional steps in themanufacturing of embodiment devices. FIGS. 8A through 17B illustratefeatures in either of the n-type region 50N and the p-type region 50P.For example, the structures illustrated in FIGS. 8A through 17B may beapplicable to both the n-type region 50N and the p-type region 50P.Differences (if any) in the structures of the n-type region 50N and thep-type region 50P are described in the text accompanying each figure.

In FIGS. 8A and 8B, the mask layer 64 (see FIG. 7 ) may be patternedusing acceptable photolithography and etching techniques to form masks74. The pattern of the masks 74 then may be transferred to the dummygate layer 62. In some embodiments (not illustrated), the pattern of themasks 74 may also be transferred to the dummy dielectric layer 60 by anacceptable etching technique to form dummy gates 72. The dummy gates 72cover respective channel regions 58 of the fins 52. The pattern of themasks 74 may be used to physically separate each of the dummy gates 72from adjacent dummy gates. The dummy gates 72 may also have a lengthwisedirection substantially perpendicular to the lengthwise direction ofrespective epitaxial fins 52.

Further in FIGS. 8A and 8B, gate seal spacers 80 can be formed onexposed surfaces of the dummy gates 72, the masks 74, and/or the fins52. A thermal oxidation or a deposition followed by an anisotropic etchmay form the gate seal spacers 80. The gate seal spacers 80 may beformed of silicon oxide, silicon nitride, silicon oxynitride, or thelike.

After the formation of the gate seal spacers 80, implants for lightlydoped source/drain (LDD) regions (not explicitly illustrated) may beperformed. In the embodiments with different device types, similar tothe implants discussed above in FIG. 6 , a mask, such as a photoresist,may be formed over the n-type region 50N, while exposing the p-typeregion 50P, and appropriate type (e.g., p-type) impurities may beimplanted into the exposed fins 52 in the p-type region 50P. The maskmay then be removed. Subsequently, a mask, such as a photoresist, may beformed over the p-type region 50P while exposing the n-type region 50N,and appropriate type impurities (e.g., n-type) may be implanted into theexposed fins 52 in the n-type region 50N. The mask may then be removed.The n-type impurities may be the any of the n-type impurities previouslydiscussed, and the p-type impurities may be the any of the p-typeimpurities previously discussed. The lightly doped source/drain regionsmay have a concentration of impurities of from about 10¹⁵ cm⁻³ to about10¹⁹ cm⁻³. An anneal may be used to repair implant damage and toactivate the implanted impurities.

In FIGS. 9A and 9B, gate spacers 86 are formed on the gate seal spacers80 along sidewalls of the dummy gates 72 and the masks 74. The gatespacers 86 may be formed by conformally depositing an insulatingmaterial and subsequently anisotropically etching the insulatingmaterial. The insulating material of the gate spacers 86 may be siliconoxide, silicon nitride, silicon oxynitride, silicon carbonitride, acombination thereof, or the like.

It is noted that the above disclosure generally describes a process offorming spacers and LDD regions. Other processes and sequences may beused. For example, fewer or additional spacers may be utilized,different sequence of steps may be utilized (e.g., the gate seal spacers80 may not be etched prior to forming the gate spacers 86, yielding“L-shaped” gate seal spacers, spacers may be formed and removed, and/orthe like. Furthermore, the n-type and p-type devices may be formed usinga different structures and steps. For example, LDD regions for n-typedevices may be formed prior to forming the gate seal spacers 80 whilethe LDD regions for p-type devices may be formed after forming the gateseal spacers 80.

In FIGS. 9A, 9B, 10A, 10B, 10C, 10D, 11A, 11B, 11C, and 11D source/drainregions 82 are formed in the fins 52 to exert stress in the respectivechannel regions 58, thereby improving performance. The source/drainregions 82 are formed in the fins 52 such that each dummy gate 72 isdisposed between respective neighboring pairs of the source/drainregions 82. In some embodiments the source/drain regions 82 may extendinto, and may also penetrate through, the fins 52. In some embodiments,the gate spacers 86 are used to separate the source/drain regions 82from the dummy gates 72 by an appropriate lateral distance so that thesource/drain regions 82 do not short out subsequently formed gates ofthe resulting FinFETs.

The formation of the source/drain regions 82 may be formed by distinctprocesses, such that the source/drain regions 82 may be differentmaterials in each region and may be formed by distinct processes.Various masking steps may be used to mask and expose appropriate regionswhen using distinct processes.

Referring first to FIGS. 9A and 9B, a patterning process is performed onthe fins 52 to form recesses 85 in source/drain regions of the fins 52.The patterning process may be performed in a manner that the recesses 85are formed between neighboring dummy gate stacks 72/74 (in interiorregions of the fins 52), or between an isolation region 56 and adjacentdummy gate stacks 72/74 (in end regions of the fins 52). In someembodiments, the patterning process may include a suitable anisotropicdry etching process, while using the dummy gate stacks 72/74, the gatespacers 86, and/or isolation regions 56 as a combined mask. The suitableanisotropic dry etching process may include a reactive ion etch (RIE),neutral beam etch (NBE), the like, or a combination thereof. In someembodiments where the RIE is used in the first patterning process,process parameters such as, for example, a process gas mixture, avoltage bias, and an RF power may be chosen such that etching ispredominantly performed using physical etching, such as ion bombardment,rather than chemical etching, such as radical etching through chemicalreactions. In some embodiments, a voltage bias may be increased toincrease energy of ions used in the ion bombardment process and, thus,increase a rate of physical etching. Since, the physical etching inanisotropic in nature and the chemical etching is isotropic in nature,such an etching process has an etch rate in the vertical direction thatis greater than an etch rate in the lateral direction. In someembodiments, the anisotropic etching process may be performed using aprocess gas mixture including fluoromethane, methane, hydrogen bromide,oxygen, argon, a combination thereof, or the like. In some embodiments,the patterning process forms recesses 85 having U-shaped bottomsurfaces. The recesses 85 may also be referred to as U-shaped recesses85, an example recess 85 of which is shown in FIG. 9B. In someembodiments, the depth of the recesses 85 is in a range from about 35 nmto about 60 nm as measure from a top surface of the fin 52.

In FIGS. 10A through 10D and 11A through 11D, the source/drain regions82 (including layers 82A and 82B) are formed in the recesses 85. InFIGS. 10A through 10D, an epitaxial layer 82A of the source/drainregions 82 is grown in the recesses 85.

The epitaxial source/drain regions 82 in the n-type region 50N may beformed by masking the p-type region 50P and etching source/drain regionsof the fins 52 in the n-type region 50N to form recesses in the fins 52.Then, the epitaxial source/drain regions 82 in the n-type region 50N areepitaxially grown in the recesses. The epitaxial source/drain layer 82Amay include any acceptable material, such as appropriate for n-typeFinFETs. For example, if the fin 52 is silicon, the epitaxialsource/drain layer 82A in the n-type region 50N may include materialsexerting a tensile strain in the channel region 58, such as silicon,silicon carbide, phosphorous doped silicon carbide, silicon phosphide,or the like. The epitaxial source/drain layer 82A in the n-type region50N may have surfaces raised from respective surfaces of the fins 52 andmay have facets.

The epitaxial source/drain regions 82 in the p-type region 50P may beformed by masking the n-type region 50N and etching source/drain regionsof the fins 52 in the p-type region 50P to form recesses in the fins 52.Then, the epitaxial source/drain regions 82 in the p-type region 50P areepitaxially grown in the recesses. The epitaxial source/drain layer 82Amay include any acceptable material, such as appropriate for p-typeFinFETs. For example, if the fin 52 is silicon, the epitaxialsource/drain layer 82A in the p-type region 50P may comprise materialsexerting a compressive strain in the channel region 58, such assilicon-germanium, boron doped silicon-germanium, germanium, germaniumtin, or the like. The epitaxial source/drain layer 82A in the p-typeregion 50P may have surfaces raised from respective surfaces of the fins52 and may have facets.

In the embodiments with boron doped silicon-germanium source/drain layer82A in the p-type region 50P, the source/drain layer 82A can beepitaxially grown with the following conditions: temperature in a rangefrom 580° C. to 630° C.; pressure in a range from 17 to 25 torr,dichlorosilane (DCS) gas at a flow rate in a range from 30 sccm to 60sccm; GeH₄ gas at a flow rate in a range from 400 sccm to 800 sccm; andB₂H₆ gas at a flow rate in a range from 40 sccm to 150 sccm. Thesource/drain epitaxial layer 82A may have a dopant concentration in arange from 10²⁰ cm⁻³ to 10²¹ cm⁻³. In the embodiments with boron dopedsilicon-germanium source/drain layer 82A in the p-type region 50P, thesource/drain epitaxial layer 82A may have a boron concentration in arange from 10²⁰ cm⁻³ to 10²¹ cm⁻³.

In FIGS. 11A through 11D, an outer source/drain epitaxial layer 82B isformed over the epitaxial layer 82A. In some embodiments, the outersource/drain epitaxial layer 82B is an ultra-heavily doped source/drainlayer 82B and has a higher dopant concentration than the layer 82A. Theouter source/drain epitaxial layer 82B may have a dopant concentrationin a range from 10²¹ cm⁻³ to 10²² cm⁻³. In the embodiments with borondoped silicon-germanium source/drain layer 82B in the p-type region 50P,the outer source/drain epitaxial layer 82B may have a boronconcentration in a range from 10²¹ cm⁻³ to 10²² cm⁻³. In someembodiments, the outer source/drain epitaxial layer 82B has from two toten times greater boron concentration than the source/drain epitaxiallayer 82A. In some embodiments, the source/drain region layers 82A and82B may be in situ doped during growth. In some embodiments, thesource/drain region layers 82A and 82B may be doped by an implantprocess. In both in situ doping and implant doping, the process may befollowed by an anneal process.

In some embodiment, the ultra-heavily doped source/drain layer 82B maybe grown to have a thickness T1. In some embodiments, the thickness T1is in a range from 5 nm to 30 nm. If the ultra-heavily dopedsource/drain layer 82B is thicker than 30 nm, defects such as nodulescan appear. These defects can affect back-end loop and degrade waferacceptance testing performance. If the ultra-heavily doped source/drainlayer 82B is thinner than 5 nm, the performance of the device will belimited as the current of the source/drain region 82 will not haveenough space to transfer. As illustrated in FIGS. 10B and 11B, both ofthe layers 82A and 82B physically contact the gate spacers 86. In someembodiments, the layer 82B contacts the gate spacer 86 but the layer 82Adoes not contact the gate spacer 86.

In the embodiments with boron ultra-heavily doped silicon-germaniumsource/drain layer 82B in the p-type region 50P, the ultra-heavily dopedsilicon-germanium source/drain layer 82B can be epitaxially grown withthe following conditions: temperature in a range from 580° C. to 630°C.; pressure in a range from 17 to 25 torr, dichlorosilane (DCS) gas ata flow rate in a range from 30 sccm to 60 sccm; GeH₄ gas at a flow ratein a range from 400 sccm to 800 sccm; and B₂H₆ gas at a flow rate in arange from 40 sccm to 150 sccm. In some embodiments, the growthconditions for the layer 82B is the same as the conditions for the layer82A, except that the growth process for the ultra-heavily dopedsilicon-germanium source/drain layer 82B is shorter than the growthprocess for the highly doped silicon-germanium source/drain layer 82A.For example, the shorter process for outer layer 82B can cause the B₂H₆gas ramp rate to have a greater ramp slope, which can cause a large B₂H₆gas flow into the growth chamber and forms the ultra-heavily dopedsilicon-germanium source/drain layer 82B.

In some embodiments, the source/drain regions 82 in the p-type regions50P include both the layers 82A and 82B while the source/drain regions82 in the n-type regions do not include the ultra-heavily dopedsource/drain layer 82B. In some embodiments, both of the regions 50P and50N include the same number of layers in the source/drain regions 82.

The above-described method of forming the source/drain regions 82 canimprove the performance of FinFET devices by reducing the parasiticresistance of the source/drain regions 82 of the semiconductor device.In the disclosed embodiments, the source/drain regions 82 include anultra-heavily doped layer 82B on the outside of the source/drain region82, which can reduce the parasitic resistance of the source/drain region82. For example, for p-type MOSFETs, such as p-type FinFETs, thesource/drain region 82 includes an outer layer ultra-heavily doped withboron to reduce the parasitic resistance. With the inclusion of theultra-heavily doped layer, the Schottky barrier height at the interfacebetween a subsequently formed conductive contact and the source/drainregion 82 is reduced, and thus, a carrier can more easily tunnel throughthe interface, which lowers the parasitic resistance. This reduction ofthe parasitic capacitance can boost the performance of the semiconductordevice. Specifically, the inventors found that for p-type MOSFETs, thep-type doping concentration in the source/drain region greatly affectsthe total resistance of the semiconductor device as the parasiticresistance is a large portion of the total resistance for p-typeMOSFETs.

As a result of the epitaxy processes used to form the epitaxialsource/drain regions 82 in the n-type region 50N and the p-type region50P, upper surfaces of the epitaxial source/drain regions have facetswhich expand laterally outward beyond sidewalls of the fins 52. In someembodiments, these facets cause adjacent source/drain layers 82A of asame FinFET to merge as illustrated by FIGS. 10C and 11C. In someembodiments, these facets cause adjacent source/drain layers 82B of asame FinFET to merge while the layers 82A are not merged. In theseembodiments, the outer layer 82B spans between adjacent layers 82A. Inother embodiments, adjacent source/drain regions 82 remain separatedafter the epitaxy process is completed as illustrated by FIGS. 10D and11D. In the embodiments illustrated in FIGS. 10C-D and 11C-D, gatespacers 86 are formed covering a portion of the sidewalls of the fins 52that extend above the STI regions 56 thereby blocking the epitaxialgrowth. In some other embodiments, the spacer etch used to form the gatespacers 86 may be adjusted to remove the spacer material to allow theepitaxially grown region to extend to the surface of the STI region 56.In some embodiments, the source/drain regions 82 have top surfaces thatare flat within process variations (see FIGS. 10C-D and 11C-D). In someembodiments, the source/drain regions 82 have faceted top surfaces.

In FIGS. 12A and 12B, a first interlayer dielectric (ILD) 88 isdeposited over the structure illustrated in FIGS. 11A and 11B. The firstILD 88 may be formed of a dielectric material, and may be deposited byany suitable method, such as CVD, plasma-enhanced CVD (PECVD), or FCVD.Dielectric materials may include phospho-silicate glass (PSG),boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG),undoped silicate glass (USG), or the like. Other insulation materialsformed by any acceptable process may be used. In some embodiments, acontact etch stop layer (CESL) 87 is disposed between the first ILD 88and the epitaxial source/drain regions 82, the masks 74, and the gatespacers 86. The CESL 87 may comprise a dielectric material, such as,silicon nitride, silicon oxide, silicon oxynitride, or the like, havinga lower etch rate than the material of the overlying first ILD 88.

In FIGS. 13A and 13B, a planarization process, such as a CMP, may beperformed to level the top surface of the first ILD 88 with the topsurfaces of the dummy gates 72 or the masks 74. The planarizationprocess may also remove the masks 74 on the dummy gates 72, and portionsof the gate seal spacers 80 and the gate spacers 86 along sidewalls ofthe masks 74. After the planarization process, top surfaces of the dummygates 72, the gate seal spacers 80, the gate spacers 86, and the firstILD 88 are level. Accordingly, the top surfaces of the dummy gates 72are exposed through the first ILD 88. In some embodiments, the masks 74may remain, in which case the planarization process levels the topsurface of the first ILD 88 with the top surfaces of the top surface ofthe masks 74.

In FIGS. 14A and 14B, the dummy gates 72, and the masks 74 if present,are removed in an etching step(s), so that recesses 90 are formed.Portions of the dummy dielectric layer 60 in the recesses 90 may also beremoved. In some embodiments, only the dummy gates 72 are removed andthe dummy dielectric layer 60 remains and is exposed by the recesses 90.In some embodiments, the dummy dielectric layer 60 is removed fromrecesses 90 in a first region of a die (e.g., a core logic region) andremains in recesses 90 in a second region of the die (e.g., aninput/output region). In some embodiments, the dummy gates 72 areremoved by an anisotropic dry etch process. For example, the etchingprocess may include a dry etch process using reaction gas(es) thatselectively etch the dummy gates 72 with little or no etching of thefirst ILD 88 or the gate spacers 86. Each recess 90 exposes and/oroverlies a channel region 58 of a respective fin 52. Each channel region58 is disposed between neighboring pairs of the epitaxial source/drainregions 82. During the removal, the dummy dielectric layer 60 may beused as an etch stop layer when the dummy gates 72 are etched. The dummydielectric layer 60 may then be optionally removed after the removal ofthe dummy gates 72.

In FIGS. 15A and 15B, gate dielectric layers 92 and gate electrodes 94are formed for replacement gates. FIG. 15C illustrates a detailed viewof region 89 of FIG. 15B. Gate dielectric layers 92 one or more layersdeposited in the recesses 90, such as on the top surfaces and thesidewalls of the fins 52 and on sidewalls of the gate seal spacers80/gate spacers 86. The gate dielectric layers 92 may also be formed onthe top surface of the first ILD 88. In some embodiments, the gatedielectric layers 92 comprise one or more dielectric layers, such as oneor more layers of silicon oxide, silicon nitride, metal oxide, metalsilicate, or the like. For example, in some embodiments, the gatedielectric layers 92 include an interfacial layer of silicon oxideformed by thermal or chemical oxidation and an overlying high-kdielectric material, such as a metal oxide or a silicate of hafnium,aluminum, zirconium, lanthanum, manganese, barium, titanium, lead, andcombinations thereof. The gate dielectric layers 92 may include adielectric layer having a k value greater than about 7.0. The formationmethods of the gate dielectric layers 92 may include Molecular-BeamDeposition (MBD), ALD, PECVD, and the like. In embodiments whereportions of the dummy gate dielectric 60 remains in the recesses 90, thegate dielectric layers 92 include a material of the dummy gatedielectric 60 (e.g., SiO₂).

The gate electrodes 94 are deposited over the gate dielectric layers 92,respectively, and fill the remaining portions of the recesses 90. Thegate electrodes 94 may include a metal-containing material such astitanium nitride, titanium oxide, tantalum nitride, tantalum carbide,cobalt, ruthenium, aluminum, tungsten, combinations thereof, ormulti-layers thereof. For example, although a single layer gateelectrode 94 is illustrated in FIG. 15B, the gate electrode 94 maycomprise any number of liner layers 94A, any number of work functiontuning layers 94B, and a fill material 94C as illustrated by FIG. 15C.After the filling of the recesses 90, a planarization process, such as aCMP, may be performed to remove the excess portions of the gatedielectric layers 92 and the material of the gate electrodes 94, whichexcess portions are over the top surface of the ILD 88. The remainingportions of material of the gate electrodes 94 and the gate dielectriclayers 92 thus form replacement gates of the resulting FinFETs. The gateelectrodes 94 and the gate dielectric layers 92 may be collectivelyreferred to as a “gate stack.” The gate and the gate stacks may extendalong sidewalls of a channel region 58 of the fins 52.

The formation of the gate dielectric layers 92 in the n-type region 50Nand the p-type region 50P may occur simultaneously such that the gatedielectric layers 92 in each region are formed from the same materials,and the formation of the gate electrodes 94 may occur simultaneouslysuch that the gate electrodes 94 in each region are formed from the samematerials. In some embodiments, the gate dielectric layers 92 in eachregion may be formed by distinct processes, such that the gatedielectric layers 92 may be different materials, and/or the gateelectrodes 94 in each region may be formed by distinct processes, suchthat the gate electrodes 94 may be different materials. Various maskingsteps may be used to mask and expose appropriate regions when usingdistinct processes.

In FIGS. 16A and 16B, a gate mask 96 is formed over the gate stack(including a gate dielectric layer 92 and a corresponding gate electrode94), and the gate mask may be disposed between opposing portions of thegate spacers 86. In some embodiments, forming the gate mask 96 includesrecessing the gate stack so that a recess is formed directly over thegate stack and between opposing portions of gate spacers 86. A gate mask96 comprising one or more layers of dielectric material, such as siliconnitride, silicon oxynitride, or the like, is filled in the recess,followed by a planarization process to remove excess portions of thedielectric material extending over the first ILD 88.

As also illustrated in FIGS. 16A and 16B, a second ILD 108 is depositedover the first ILD 88. In some embodiments, the second ILD 108 is aflowable film formed by a flowable CVD method. In some embodiments, thesecond ILD 108 is formed of a dielectric material such as PSG, BSG,BPSG, USG, or the like, and may be deposited by any suitable method,such as CVD and PECVD. The subsequently formed gate contacts 110 (FIGS.17A and 17B) penetrate through the second ILD 108 and the gate mask 96to contact the top surface of the recessed gate electrode 94.

In FIGS. 17A and 17B, gate contacts 110 and source/drain contacts 112are formed through the second ILD 108 and the first ILD 88 in accordancewith some embodiments. Openings for the source/drain contacts 112 areformed through the first and second ILDs 88 and 108, and openings forthe gate contact 110 are formed through the second ILD 108 and the gatemask 96. The openings may be formed using acceptable photolithographyand etching techniques. A liner (not shown), such as a diffusion barrierlayer, an adhesion layer, or the like, and a conductive material areformed in the openings. The liner may include titanium, titaniumnitride, tantalum, tantalum nitride, or the like. The conductivematerial may be copper, a copper alloy, silver, gold, tungsten, cobalt,aluminum, nickel, or the like. A planarization process, such as a CMP,may be performed to remove excess material from a surface of the ILD108. The remaining liner and conductive material form the source/draincontacts 112 and gate contacts 110 in the openings. An anneal processmay be performed to form a silicide at the interface between theepitaxial source/drain regions 82 and the source/drain contacts 112. Thesource/drain contacts 112 are physically and electrically coupled to theepitaxial source/drain regions 82, and the gate contacts 110 arephysically and electrically coupled to the gate electrodes 106. Thesource/drain contacts 112 and gate contacts 110 may be formed indifferent processes, or may be formed in the same process. Althoughshown as being formed in the same cross-sections, it should beappreciated that each of the source/drain contacts 112 and gate contacts110 may be formed in different cross-sections, which may avoid shortingof the contacts.

In some embodiments, the source/drain contacts 112 physically contactthe ultra-heavily doped source/drain layer 82B. In some embodiments, thesource/drain contacts 112 physically contact the flat top surface of theultra-heavily doped source/drain layer 82B (see FIGS. 11C-D) (see FIG.18 ).

FIG. 18 is a perspective view of a source/drain region 82 in accordancewith some embodiments. As discussed above, the ultra-heavily dopedsource/drain layer 82B, the Schottky barrier height at the interfacebetween the conductive contact 112 and the source/drain region 82 isreduced, and thus, a carrier can more easily tunnel through theinterface, which lowers the parasitic resistance. FIG. 18 graphicallyillustrates this idea by showing that the current 120 passes along allportions—upper, middle, and lower—of the layer 82B. This currentcarrying ability lowers the parasitic resistance of the source/drainregions 82 and improves the performance of the device.

FIG. 19 is a graph illustrating concentration of dopant of asource/drain region in accordance with some embodiments. Specifically,the line 130 illustrates the doping concentration of the ultra-heavilyouter source/drain layer 82B and the line 132 illustrates the dopingconcentration of the source/drain layer 82A. The values illustrated inFIG. 19 are an example, but other values are also within the scope ofthe disclosure. As illustrated in FIG. 19 , the layer 82B has threepeaks of dopant concentration: a first peak at a depth between 0 nm and5 nm, a second peak at a depth between 10 nm and 15 nm, and a third peakat a depth between 20 nm and 25 nm. In the illustrated embodiment, thefirst and third peaks are greater than the second peak, but otherconfigurations are also within the scope of the disclosure.

Embodiments may achieve advantages. The disclosed semiconductor deviceand method of forming the same improve the performance of FinFET devicesby reducing the parasitic resistance of the source/drain regions of thesemiconductor device. In the disclosed embodiments, the source/drainregions include an ultra-heavily doped layer on the outside of thesource/drain region, which can reduce the parasitic resistance of thesource/drain region. For example, for p-type MOSFETs, such as p-typeFinFETs, the source/drain region includes an outer layer ultra-heavilydoped with boron to reduce the parasitic resistance. With the inclusionof the ultra-heavily doped layer, the Schottky barrier height at theinterface between a conductive contact and the source/drain region isreduced, and thus, a carrier can more easily tunnel through theinterface, which lowers the parasitic resistance. This reduction of theparasitic capacitance can boost the performance of the semiconductordevice. Specifically, the inventors found that for p-type MOSFETs, thep-type doping concentration in the source/drain region greatly affectsthe total resistance of the semiconductor device as the parasiticresistance is a large portion of the total resistance for p-typeMOSFETs. The disclosed processes and structures can improve theperformance and reliability of the FinFET device.

The disclosed FinFET embodiments could also be applied to nanostructuredevices such as nanostructure (e.g., nanosheet, nanowire,gate-all-around, or the like) field effect transistors (NSFETs). In anNSFET embodiment, the fins are replaced by nanostructures formed bypatterning a stack of alternating layers of channel layers andsacrificial layers. Dummy gate stacks and source/drain regions areformed in a manner similar to the above-described embodiments. After thedummy gate stacks are removed, the sacrificial layers can be partiallyor fully removed in channel regions. The replacement gate structures areformed in a manner similar to the above-described embodiments, thereplacement gate structures may partially or completely fill openingsleft by removing the sacrificial layers, and the replacement gatestructures may partially or completely surround the channel layers inthe channel regions of the NSFET devices. ILDs and contacts to thereplacement gate structures and the source/drain regions may be formedin a manner similar to the above-described embodiments. A nanostructuredevice can be formed as disclosed in U.S. Patent Application PublicationNo. 2016/0365414, which is incorporated herein by reference in itsentirety.

In an embodiment, a device includes a first fin extending from asubstrate. The device also includes a first gate stack over and alongsidewalls of the first fin. The device also includes a first gate spacerdisposed along a sidewall of the first gate stack. The device alsoincludes and a first source/drain region in the first fin and adjacentthe first gate spacer, the first source/drain region including a firstepitaxial layer on the first fin, the first epitaxial layer having afirst dopant concentration of boron. The device also includes and asecond epitaxial layer on the first epitaxial layer, the secondepitaxial layer having a second dopant concentration of boron, thesecond dopant concentration being greater than the first dopantconcentration.

Embodiments may include one or more of the following features. Thedevice where the second epitaxial layer has a thickness in a range from5 nm to 30 nm. The second epitaxial layer has a flat top surface. Thedevice further including an etch stop layer over the first source/drainregion and on a sidewall of the first gate spacer, a first interlayerdielectric over the etch stop layer, a second interlayer dielectric overthe first interlayer dielectric, and a first conductive contactextending through the first and second interlayer dielectrics and theetch stop layer, the first conductive contact being electrically coupledto the first source/drain region. The second epitaxial layer contactsthe first gate spacer. The first epitaxial layer contacts the first gatespacer. The device further including a second fin extending from asubstrate, a second gate stack over and along sidewalls of the secondfin, a second gate spacer disposed along a sidewall of the second gatestack, and a second source/drain region in the second fin and adjacentthe second gate spacer, the second source/drain region including a thirdepitaxial layer, the third epitaxial layer having a different materialcomposition than the first and second epitaxial layers. The seconddopant concentration is from two to ten times greater than the firstdopant concentration.

In an embodiment, a method includes depositing a first dummy gate overand along sidewalls of a first fin, the first fin extending from asubstrate. The method also includes forming a first gate spacer along asidewall of the first dummy gate. The method also includes forming afirst recess in the first fin adjacent the first gate spacer. The methodalso includes and forming a first source/drain region in the firstrecess, the forming the first source/drain region including epitaxiallygrowing a first layer in the first recess, the first layer extendingabove a top surface of the first fin, the first layer having a firstdopant concentration of a first dopant. The method also includes andepitaxially growing a second layer on the first layer, the second layerhaving a second dopant concentration of the first dopant, the seconddopant concentration being greater than the first dopant concentration.

Embodiments may include one or more of the following features. Themethod where the first layer of the first source/drain region physicallycontacts the first gate spacer. The first layer is epitaxially grownwith a first set of growth conditions for a first period of time, andwhere the second layer is epitaxially grown with the first set of growthconditions for a second period of time, the second period of time beingshorter than the first period of time. The first dopant is boron. Thesecond layer has a thickness in a range from 5 nm to 30 nm. The methodfurther including replacing the first dummy gate with a functional gatestack disposed over and along sidewalls of the first fin. The seconddopant concentration is from two to ten times greater than the firstdopant concentration. The second dopant concentration is in a range from10²¹ cm⁻³ to 10²² cm⁻³.

In an embodiment, a method includes forming a first dummy gate over andalong sidewalls of a first fin extending upwards from a substrate. Themethod also includes forming a first gate spacer along a sidewall of thefirst dummy gate. The method also includes etching a first recess in thefirst fin adjacent the first gate spacer. The method also includesforming a first source/drain region in the first recess, the firstsource/drain region including a first epitaxial layer and a secondepitaxial layer, the first epitaxial layer being grown from the firstfin in the first recess, the second epitaxial layer being grown from thefirst epitaxial layer, the second epitaxial layer having a greaterconcentration of boron than the first epitaxial layer. The method alsoincludes and replacing the first dummy gate with a first functional gatestack disposed over and along sidewalls of the first fin.

Embodiments may include one or more of the following features. Themethod where the second epitaxial layer physically contacts the firstgate spacer. The method further including forming a second dummy gateover and along sidewalls of a second fin extending upwards from thesubstrate, forming a second gate spacer along a sidewall of the seconddummy gate, etching a second recess in the second fin adjacent thesecond gate spacer, forming a second source/drain region in the secondrecess, the second source/drain region including a third epitaxiallayer, the third epitaxial layer having a different material compositionthan the first and second epitaxial layers, and replacing the seconddummy gate with a second functional gate stack disposed over and alongsidewalls of the first fin. The concentration of boron of the secondepitaxial layer is in a range from 10²¹ cm⁻³ to 10²² cm⁻³.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a first fin extending from asubstrate; a first gate stack over and along sidewalls of the first fin;a first gate spacer disposed along a sidewall of the first gate stack;and a first source/drain region in the first fin and adjacent the firstgate spacer, the first source/drain region comprising: a first epitaxiallayer on the first fin, the first epitaxial layer having a first dopantconcentration of boron; and a second epitaxial layer on the firstepitaxial layer, the second epitaxial layer having a second dopantconcentration of boron, the second dopant concentration being greaterthan the first dopant concentration, the second epitaxial layer being anoutermost layer of the first source/drain region.
 2. The device of claim1, wherein the second epitaxial layer has a thickness in a range from 5nm to 30 nm.
 3. The device of claim 1, wherein the second epitaxiallayer has a flat top surface.
 4. The device of claim 1 furthercomprising: an etch stop layer over the first source/drain region and ona sidewall of the first gate spacer, the etch stop layer physicallycontacting the second epitaxial layer of the first source/drain region;a first interlayer dielectric over the etch stop layer; a secondinterlayer dielectric over the first interlayer dielectric; and a firstconductive contact extending through the first and second interlayerdielectrics and the etch stop layer, the first conductive contact beingelectrically coupled to the first source/drain region.
 5. The device ofclaim 1, wherein the second epitaxial layer contacts the first gatespacer.
 6. The device of claim 5, wherein the first epitaxial layercontacts the first gate spacer.
 7. The device of claim 1 furthercomprising: a second fin extending from a substrate; a second gate stackover and along sidewalls of the second fin; a second gate spacerdisposed along a sidewall of the second gate stack; and a secondsource/drain region in the second fin and adjacent the second gatespacer, the second source/drain region comprising a third epitaxiallayer, the third epitaxial layer having a different material compositionthan the first and second epitaxial layers.
 8. The device of claim 1,wherein the second dopant concentration is from two to ten times greaterthan the first dopant concentration.
 9. A method comprising: depositinga first dummy gate over and along sidewalls of a first fin, the firstfin extending from a substrate; forming a first gate spacer along asidewall of the first dummy gate; forming a first recess in the firstfin adjacent the first gate spacer; forming a first source/drain regionin the first recess, the forming the first source/drain regioncomprising: epitaxially growing a first layer in the first recess, thefirst layer extending above a top surface of the first fin, the firstlayer having a first dopant concentration of a first dopant; andepitaxially growing a second layer on the first layer, the second layerhaving a second dopant concentration of the first dopant, the seconddopant concentration being greater than the first dopant concentration;and forming an etch stop layer over the first source/drain region and ona sidewall of the first gate spacer, the etch stop layer extending alonga top surface of and physically contacting the second layer of the firstsource/drain region.
 10. The method of claim 9, wherein the first layerof the first source/drain region physically contacts the first gatespacer.
 11. The method of claim 9, wherein the first layer isepitaxially grown with a first set of growth conditions for a firstperiod of time, and wherein the second layer is epitaxially grown withthe first set of growth conditions for a second period of time, thesecond period of time being shorter than the first period of time. 12.The method of claim 9, wherein the first dopant is boron.
 13. The methodof claim 9, wherein the second layer has a thickness in a range from 5nm to 30 nm.
 14. The method of claim 9 further comprising: replacing thefirst dummy gate with a functional gate stack disposed over and alongsidewalls of the first fin.
 15. The method of claim 9, wherein thesecond dopant concentration is from two to ten times greater than thefirst dopant concentration.
 16. The method of claim 9, wherein thesecond layer is an outermost layer of the first source/drain region. 17.A method comprising: forming a first dummy gate over and along sidewallsof a first fin extending upwards from a substrate; forming a first gatespacer along a sidewall of the first dummy gate; etching a first recessin the first fin adjacent the first gate spacer; forming a firstsource/drain region in the first recess, the first source/drain regioncomprising a first epitaxial layer and a second epitaxial layer, thefirst epitaxial layer being grown from the first fin in the firstrecess, the second epitaxial layer being grown from the first epitaxiallayer, the second epitaxial layer having a greater concentration ofboron than the first epitaxial layer, the second epitaxial layer beingan outermost layer of the first source/drain region; and replacing thefirst dummy gate with a first functional gate stack disposed over andalong sidewalls of the first fin.
 18. The method of claim 17, whereinthe second epitaxial layer physically contacts the first gate spacer.19. The method of claim 17 further comprising: forming a second dummygate over and along sidewalls of a second fin extending upwards from thesubstrate; forming a second gate spacer along a sidewall of the seconddummy gate; etching a second recess in the second fin adjacent thesecond gate spacer; forming a second source/drain region in the secondrecess, the second source/drain region comprising a third epitaxiallayer, the third epitaxial layer having a different material compositionthan the first and second epitaxial layers; and replacing the seconddummy gate with a second functional gate stack disposed over and alongsidewalls of the first fin.
 20. The method of claim 17 furthercomprising: forming a first conductive contact over and electricallycoupled to the first source/drain region, the first conductive contactphysically contacting the second epitaxial layer of the firstsource/drain region.